HOW TO FIRE A BISCUIT OR GLOST OVEN
Simply put, firing an updraught oven, downdraught oven and muffle kiln involved three main procedures:
- Placing
- Firing
- Drawing
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| Gladstone China, Longton Biscuit ovens firing timetable. Oven No.1 and No.2 Photo: from a display at Gladstone Pottery Museum |
In more detail the process includes:
- The bottle oven's firing chamber was filled (placed, or set) with pottery which had been placed, for protection, in saggars.
- The entrance doorway to the firing chamber, called the wicket, was built up and sealed, forming the clammins.
- The temperature inside the firing chamber was raised to over 1000C to fire the pottery. Coal or oil was used as the fuel.
- After the peak firing conditions of temperature and 'soak' had been achieved, the oven was allowed to cool.
- The clammins was broken down and the fired pottery taken out.
- The whole process would then start all over again.
Placing
This was the first step in firing an oven. Pottery was placed into saggars but different techniques were required depending on whether the pottery was for a biscuit or a glost firing.
For the biscuit firing of green (dry clay) holloware, the bottom of the saggar was covered with a thin layer of silica sand or powdered flint. Then as many pieces as possible were packed into the saggar with extra sand or flint as required.
Green flatware, however, was bedded. A dry clay pot was placed in the saggar onto a layer of silica sand or flint, then covered with a layer of sand or flint followed by another pot then another layer of sand or flint and so on until the saggar was full. This method reduced the problem of the ware warping during the intense heat of the firing.
For the glost firing of dipped biscuit ware all the pieces had to be kept separate otherwise, during the firing, when the glaze melted they would fuse together. To separate the ware a selection of kiln furniture was used. No sand or flint was used in glost firing.
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| Placing onglaze decorated flatware into saggar using cranks prior to placing in the oven Photo: Terry Woolliscroft Collection Date: Last Bottle Oven Firing August 1978 |
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| Glost placing into saggars Photos from The Last Bottle Oven Firing 1978 https://lbof1978.blogspot.com/ |
The pottery was placed in the saggars in a small room known as the placing shop or saggar house adjoining the bottle oven. Ware was brought in on wareboards which were then put on stillages above the placing benches. When the saggars were full, they were carried into the oven by the placers on their shoulders or heads.
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| Placing the oven Photos from The Last Bottle Oven Firing 1978 https://lbof1978.blogspot.com/ |
To help a placer balance a saggar on his head he would put a placer's roll into his hat. A placer's roll resembles a ring doughnut and was made out of stockings. The weight of a full saggar was approximately 0.5cwt (25kgs). An average oven held about 2,000 saggars, a big oven held 3,000 or more.
Inside the oven, the saggars were stacked in bungs. The bungs were placed systematically, filling the oven from the sides and rear first, then towards the well hole in the centre. Over the well hole a pipe bung was built, using saggars with no bottom, to allow the hot gases and smoke to escape upwards from the flues. Placing then continued towards the wicket until the oven was full. To reach the top of each bung the placers used robust wooden ladders called 'osses.
Placing the oven was a skilled job. The floor of the oven sloped upwards towards the middle so bungs had to be wedged upright using pieces of broken saggar called scotches and half bricks called half-enders. Different parts of the oven reached different temperatures so different types of ware had to be placed in different locations. Placers required a deep understanding of how the oven behaved during firing.
Firing
When the oven was full, the wicket was bricked up and daubed with a mix of clay and sand. It was then called the clammins.
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| The wicket is daubed with clay and sand over the bricks, to seal them. Photo from The Last Bottle Oven Firing 1978 Billy Ryan https://lbof1978.blogspot.com/ |
When the oven was ready to fire, coal, together with kindling of sticks and paper, was lumped into the firemouths. Fires were kindled.
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| Preparing kindling Date: c1900 |
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| Kindling in one of the firemouths to start the firing Photo: source unknown, location unknown. Date: early 1960s |
Once the fires had started the oven was baited at intervals of about 3 to 4 hours. The coal was carefully poured onto the fires with shovels or special baiting boxes. It was not thrown on haphazardly. This technique preserved the condition of the burning coals.
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| Cod placer and an oven fireman baiting Photo source: unknown Date: unknown |
At the early stages of the firing the temperature rise was kept particularly slow while the moisture in the pottery, and the brickwork of the oven itself, was driven out. This was known as the smoking period when the most black smoke was produced.
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| Smoking period, Middleport Pottery Photo: source unknown Date: unknown |
The fireman held a responsible job often regarded as the most important man on the factory. He stayed with the oven with little or no rest throughout what could be several days and nights of the firing. Sometimes he would deputise his sitter-up or night fireman to take charge while he snatched some sleep. It was essential that the fireman gave his constant attention during firing. He followed its progress by taking trials at regular intervals.
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| Fireman Jim Evans W T Copeland factory, Stoke Photo source: unknown Date: unknown |
Around the oven were regularly-spaced trial holes built into the brickwork. They were blocked from the outside with a loose brick during firing. The fireman was able to remove the brick to draw out a trial. Inside the oven the trial would have been placed in a saggar with its side knocked out. Between this saggar and the wall of the oven would be another saggar with both sides missing enabling the fireman to reach through with a hooked iron poker to draw the trial out from deep inside the oven.
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| Bullers Rings being removed from bottom of clammins Photos from The Last Bottle Oven Firing 1978 https://lbof1978.blogspot.com/ |
Originally there were no thermometers which would withstand the high temperatures reached in an oven. Most of the firing was done by guesswork. The colour of the flame was sometimes used as a guide but this proved to be unreliable. Another method was to use small samples of the ware which was being fired as trials. By observing the colour of the piece of pot the fireman would judge the progress of the firing. Josiah Wedgwood (the famous and important 18th century potter) did many secret experiments to find ways of measuring the temperature accurately. He made use of the fact that when pottery is fired, it shrinks. He fired small pieces of pottery and withdrew them from the oven at intervals and measured them on his special gauge.
It was from Wedgwood's experiments that today's pyroscopes have been developed. Still in use today (2018) these include Bullers rings which look like large Polo mints. They are made from a mixture of ceramic materials which shrink to a known degree depending on temperature and time in the oven. They are withdrawn at intervals throughout the firing and measured on a Bullers gauge. Using these rings the progress of firing can be monitored.
Other pyrometers include Seger cones which are pyramid-shaped clay pieces. These are designed to curl over at particular temperatures. Holdcroft bars work on the same principle as Seger cones but are small bars, supported at each end, which sag at particular temperatures. Optical pyrometers which depended on colour matching are sometimes used.
Fine control of the draught was achieved by altering the position of the dampers in the crown. Dampers were substantial hinged brick and iron flaps. The fireman would open and close them using a pulley system from ground level. By opening and closing different ones the draught was altered in different sections of the oven so that the fires would burn more fiercely or more gently.
After about 48 hours the required peak temperature was reached. The oven was then allowed to soak (hold its maximum temperature) for 2 or 3 hours and then the fires were left to go out.
Drawing
The last step in firing an oven was drawing (emptying). To gain entrance into the oven the clammins was broken down and the bricks removed. They were put to one side for re-use at the next firing. The oven was then left to cool. As soon as a man could enter without being harmed by the heat, the oven was drawn.
Unscrupulous factory owners, wanting their pottery urgently, made ovenmen enter ovens which were far too hot. It was dangerous and unhealthy. They wore wet rags over their hands and faces to give them at least some protection.
Once the first saggars by the wicket had been removed wooden staging was installed. 'Osses were not used during drawing. The staging was supported on saggars still inside the oven containing the fired pottery. This allowed faster and safer removal of the saggars from the tops of bungs. As drawing progressed this staging could be rearranged and then gradually removed.
Drawers worked as a team but had specific roles. The skimmer took the topmost saggars off the bungs and passed them down to men below. The second man received saggars from the skimmer and, as drawing progressed, removed saggars from the middle of bungs. The bag man worked on the floor of the oven receiving from the second man and also drawing the bottom saggars of a bung. He had the worst drawing job. He had to lift the heavy saggars from below waist level constantly bending and stretching, twisting and turning.
As fired ware was drawn it was sent to its next process and the empty saggars reused or, if they were cracked or broken, recycled as grog or scotches.
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DOWNDRAUGHT OVENS
Downdraught ovens were placed and drawn in a similar way to updraught ovens but because the hot gases from the fires were made to pass through the setting twice, the firing process and its control was different and more complex.
The downdraught was kindled in the same way as an updraught oven but when it was sufficiently hot, the crown damper was closed and dampers in the exit flues opened. By this means the heat was directed down through the setting and out through the holes in the floor of the oven into the exit flues and then out to the chimney which was either integral to, or separate from, the oven, depending n its design.
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| Downdraught ovens, Wilkinson Patent type. Former Acme Marls Works, Bournes Bank, Burslem |
A number of designs of downdraught ovens were developed and some were patented - see the 'types' page here>
MUFFLE KILNS
This type of kiln was designed to keep the open flame of the fire completely separate from the pottery. It was used to fire pottery which had been decorated with coloured enamels or precious metals such as gold and platinum. Saggars were not needed as the flames did not enter the firing chamber. Decorated pottery was placed onto refractory shelves inside the kiln. Shelves could be adjusted or removed to suit the size of the pottery being fired. Very large items, such as sanitaryware, could be freestanding within a larger muffle kiln.
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| Kindling a muffle kiln Photo source: unknown Date: unknown |
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| Muffle kiln, Furnivals, Elder Road, Cobridge. Charlie Boardman emptying the ash pit after a firing. Photo: Courtesy of Jenny Ward Date: 1930s |
Decorated pottery was fired up to a temperature of between 700°C and 800°C. Sanitaryware required higher temperatures. When cool enough the pottery was carefully drawn from the kiln.
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| Muffle kiln fireman. Photo source: unknown Date: unknown |
THE IMPORTANCE OF A SUCCESSFUL FIRING
The single most important part of the potting process is the biscuit firing. It is here that the pottery body mixed by the potter, according to a recipe, undergoes its greatest test - the ability to withstand fire and to emerge from it in the form of sound, useful, practical or beautiful, ceramic products.
Prior to firing much time and effort will have been spent on creating a mixture of materials which are relatively easily handled and fashioned into a final shape. That mixture, however, is of no commercial importance unless it can get through the oven with low wastage, and result in products possessing the properties intended by the potter. It is at this stage of the production cycle that the painstaking care used in controlling raw materials pays dividends, as even minor variations in certain of the materials can often cause ruinous kiln losses.
Prior to firing much time and effort will have been spent on creating a mixture of materials which are relatively easily handled and fashioned into a final shape. That mixture, however, is of no commercial importance unless it can get through the oven with low wastage, and result in products possessing the properties intended by the potter. It is at this stage of the production cycle that the painstaking care used in controlling raw materials pays dividends, as even minor variations in certain of the materials can often cause ruinous kiln losses.
1715 - COST OF FIRING AN OVEN
According to Josiah Wedgwood, the weekly outlay of a pottery firing one large oven of ware in 1715 was as follows:
£1.10.00 - Six men, three at 4/- and three, including the master, at 6/-
£0.05.00 - Four boys at 1/3d
£0.02.00 - Rent of works at £5 a year
£0.04.00 - Wear and tear of ovens etc at £10 a year
£1.14.00 - Clay, coal and other materials
£0.10.00 - Master’s profit (besides 6/- for his labour included above)
£4.05.00 - THE TOTAL - Four pounds and five shillings
£1.10.00 - Six men, three at 4/- and three, including the master, at 6/-
£0.05.00 - Four boys at 1/3d
£0.02.00 - Rent of works at £5 a year
£0.04.00 - Wear and tear of ovens etc at £10 a year
£1.14.00 - Clay, coal and other materials
£0.10.00 - Master’s profit (besides 6/- for his labour included above)
£4.05.00 - THE TOTAL - Four pounds and five shillings
1846 - ART AND HISTORY OF THE POTTING BUSINESS
Extracts from 'Art and History of the Potting Business' by William Evans
"Compiled from the most practical of sources for the especial use of working potters by their devoted friend William Evans. Shelton. Printed at the Examiner Office, Miles Bank."
THE potters' oven for biscuit is usually larger than that for glaze; and is, in form, a cylinder ten or twelve feet high, and from ten to fifteen feet in diameter, surmounted by a dome from three to five feet in height, in whose centre is an aperture about two feet in diameter.
WHEN the oven is filled, and all arranged, the door-way is built up with bricks, and plastered over with mud, and short iron bars are hooked into an iron frame that goes along the edge of the door-way, and is connected with two or three ranges of very strong hooking-pieces of iron, that go round, like hoops, one at the commencement of the dome, and two others on the cylinder, and which guards are necessary to prevent the high temperature of the whole causing the cylinder to crack or bilge out.
WHEN the baking is completed, the mouths are drawn (the fuel is extracted) as quickly as possible; the whole is left to cool for several hours, (the entrance is opened,) the clammins are taken down, the saggars are emptied as quickly as possible into baskets, and carried into the sorting room, where they are carefully examined, and by sounds tried whether fit for further processes.
The full text of firing a biscuit oven in 1846 is available as a downloadable pdf here>
1901 - NOTES ON THE MANUFACTURE OF EARTHENWARE
Extracts from the book by Ernest Albert Sandeman, 1901
Ovens built from the same plans vary in the treatment they require, and the best results will only be obtained by the man who has studied their idiosyncrasies. Accidents happen, bungs move, trials can neither be seen nor drawn; experience only can then guide the fireman. The colour of the interior of the oven, the appearance of the saggars and the bricks in the flues, will all be guides for him.
It is rare to fire an oven without noticing something unusual, and the successful man is he who notices these little things, and knows from past experience what is the right thing to do, and does it at once. As in so many other things an ounce of experience is worth pounds of theory, though the ideal should be the two combined. The mere getting to a certain fixed heat at the places where the trials are, is but the A-B-C of firing, which those who put their faith on trials alone, would soon find out to their cost."
1921 - PRACTICAL TREATISE on the MANUFACTURE OF POTTERY
Extracts from the book by John Gater, Consultant Potter, 1921
PREFACE
Hitherto no practical Treatise on the Manufacture of Pottery has been written, although many admirable theoretical works on the subject have been published. As the practical side abounds in innumerable details upon the observance of which depends the excellence of manufacture, the Author trusts that this work based upon an experience of many years may be of service to the industry as a whole. Every effort has been made to make it as comprehensive and explanatory as possible. Alsager, Stoke-on-Trent, August, 1921.
BISCUIT FIRING
- A fireman should make himself acquainted with every detail connected with his ovens and their peculiarities in working, for it is an open secret that although the brickwork, flues, etc., are constructed exactly the same, no two ovens work quite alike. The position of an oven, and also at times climatic conditions, make some difference to its behaviour.
- In the first place it is highly important to see that the oven is in thorough repair. No good results can be expected if the flues are partially choked, the body of the oven drawing air, or the mouths and bags in a bad state.
- It is extremely important that the temperature of all ovens be gradually raised, in fact they should be fired steadily from start to finish, otherwise there will be specked, sulphured, and crooked ware.
GLOST FIRING
- The flame must always be kept pricking over the top of the bag, so that there be no chilling either of bags or flues.
- If one mouth has burnt down lower than the others, it is better to put a lump or so on to keep it working until all mouths are ready for baiting.
- All quarters should be kept level, for if one portion of the oven is firing and the other is fired up feathering of the glaze will ensue, especially in the case of some fuels.
- It is extremely important that the oven be kept as free from sulphur as possible, anything in the nature of a reducing atmosphere should be strenuously guarded against, it sometimes causes immense damage to the ware.
The full text is available as a downloadable pdf here>
1927 - FIRING CHINA OR EARTHENWARE BISCUIT
Extracts from 'Pottery, being a simple account of the history of pottery and a description of some of the processes employed in its manufacture'
Charles Noake and Harold J Plant. 1927
As soon as the oven is filled, the clamming bricks are placed in position, and the doorway built up with loose bricks which are plastered over with sand in order to keep out the cold air. At various points round the oven, in the body of the oven itself, holes which are closed with a loose brick and sand are left so that the fireman can draw out from time to time his trials. These trials are placed in saggars having an opening, and can be readily removed with an iron rod. There is also a way left right to the centre of the oven, and here again the test pieces are so arranged to facilitate withdrawal. The trials used for china biscuit are sections of china cups, i.e. the actual body as used for the ware itself.
The firing of a china biscuit oven is usually a process which occupies about 60 hours. The first 24 hours must be carried on fairly slowly so as to drive out all moisture. If an oven is rushed at the beginning it always results in considerably more loss than usual.
The firing of biscuit earthenware ovens is very similar to the firing of china ovens, except that the final temperature required is not as high as in the case of china, the earthenware temperature being 1,100 degrees centigrade, while china requires a temperature of 1,225 to 1,250 degrees. It is this extra temperature in the case of china that increases the losses, and makes the firing process so much more
difficult. Sand, known as placing sand, of a special quality, is used instead of flint in the earthenware trade. The placing process is much more simple. Practically all cups can be successfully boxed, but even here one has to form certain definite rules for placing, and to take the same care during the firing process, or the result will be failure.
The full text is available as a downloadable pdf here>
1929 - FIRING SANITARYWARE
A short edit from the Twyfords 1929 manufacturing film showing the firing of earthenware sanitaryware in an updraught bottle oven, and fireclay in a muffle kiln. Also scenes from dipping and warehousing inspection.
1948 - TEMPERATURE MEASUREMENT
Extract from CERAMIC WHITEWARES
Rexford Newcomb, 1948
“Temperature is a measure of the intensity of heat. The firing of ceramic wares involves both time and temperature.
The ware must 'soak' for a definite period of time at the high temperature of the kiln in order to allow its interior to approach the same temperature as the surface. The more thick-walled the piece, the longer this soaking period must be. For this reason a bung of plates packed in bedding material cannot be fired as quickly as a stack which has not been bedded.
In order to check this time/temperature effect it is customary to use a ‘pyroscope’. These may be pyrometric cones or Bullers Rings. Pyrometric cones are made from a series of controlled mixes of various minerals. The cones, mounted in a small pat of fire clay, are placed in such a way that they may be observed through peep holes in the side wall of the kiln.
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| A particular form of the pyrometric cone developed by Hermann Seger in Berlin, in 1886 |
When placed at an angle 82 degrees from the horizontal, a cone will slowly bend down under continued heat. When it has bent so that its tip is on the same horizontal plane as the base, the cone is said to be 'down.' Intermediate stages are expressed by referring to the positions of the hour hand of a clock. Thus, 'Cone 10 at 3 o’clock' means that Cone 10 has slumped so that the tip is about half-way down to the base. The fusion point of any cone depends upon the rate at which the cone is heated. Different types of whiteware products are biscuit fired at temperatures ranging from about Cone 2 to Cone 04. And glazes are matured at temperatures from Cone 01 to Cone 10, or lower."
1976 - FIRING REFRACTORY KILN FURNITURE
Oil fired, Wilkinson Downdraught, stack type, bottle ovens, Acme Marls Works, Bournes Bank, Burslem - Summer 1976
When we visited this factory in July and August 1976, it supported three oil-fired Wilkinson downdraught stack ovens. They were numbered 6, 7 and 8. At the time, Number 8 was empty, Number 7 had been placed (set-in, or filled) and Number 6 was being fired.
When we visited this factory in July and August 1976, it supported three oil-fired Wilkinson downdraught stack ovens. They were numbered 6, 7 and 8. At the time, Number 8 was empty, Number 7 had been placed (set-in, or filled) and Number 6 was being fired.
The products being fired were refractory 'bats' or kiln furniture, for use in the pottery industry. We were privileged to have been there and able to take just a few photos of the firing of No.6.
Oven No.7 was fired sometime later and was the last oven to be fired at the factory before manufacture was transferred to its new site in the town. We were told that a ceremonial flaming torch was to be taken from the fire in oven No.7 to the new gas kiln at the new site.
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| Acme Marls, Burslem July/August 1976 Photos: Terry Woolliscroft Collection |
More pics of the firing in July/August 1976 here>
1951 - THE POTTERY OVEN by Robert Copeland
from a contemporary record of a biscuit oven firing of the Big Oven at Spode Factory, Stoke, 21st June 1951
The ordeal by fire is of the greatest importance in any pottery manufacture. Famous factories have closed because the body formula permitted such a narrow range of safety in firing that only two or three unsuccessfully fired ovens meant utter ruin to the master potter.
Today, big factories fire their products in continuous tunnel kilns, carefully controlled by instruments and safety devices, but in earlier times intermittently fired ovens were used. In North Staffordshire, the English Potteries, these ovens were surrounded by a brick hovel to protect the oven from the weather and to enable the fireman to control the draught to the fires: because of their characteristic shape they are known as 'bottle ovens'.
Round ovens of one type or another were universally used for firing pottery since very early times: the basic structure is a round oven or kiln with firemouths around the outside whose flues emerge inside the oven to carry the flames, to heat the load, or 'setting' of pottery.
When larger ovens were built and higher temperatures required (for example, in stoneware production) the control of the draught of air became more important, so a hovel was built above the oven to increase the draught as in a chimney; various holes were left in the walls of the roof, or 'crown' of the oven, through which the air was controlled by dampers.
It was in 1960 that the last firing of a bottle oven on the Spode Works [in Stoke] took place; it was a china biscuit oven that is, the bone china clayware was fired to about 1260°C. to the 'biscuit' stage and the four ovens in use up until then were replaced with a gas-fired tunnel kiln. In order to explain the firing process, I shall describe first the structure of the up-draught oven secondly, the 'setting in' of the oven with saggars, including a note on 'placing' the ware; thirdly, the firing schedule: this account will relate to the firing of the No.4 oven, the 'Big' Oven, in January 1951.
The full text is available as a downloadable pdf here>
2009 - THE POTTERY BOTTLE OVEN and FIRING PROCESS
Extracts courtesy of Robert Copeland, from his book 'Manufacturing Processes of Tableware during the Eighteenth & Nineteenth Centuries.' Published 2009 from a contemporary record date 21 June 1951
“The ordeal by fire is of the greatest importance in any pottery manufacture. Pottery factories have closed because the body formula permitted such a narrow range of safety in firing that it took only two or three unsuccessfully fired ovens to cause bankruptcy. ...when admiring a piece of fine pottery, spare a grateful thought for the fireman.”
The full text is available as a downloadable pdf here>